Article
Features of the tunneling conductance of break-junctions in underdoped BaFe2-xNixAs2 single crystals
I. A. Nikitchenkov
Faculty of Physics, Lomonosov Moscow State University, Leninskie Gory, 1, 119991, Moscow, Russia
P. N. Lebedev Physical Institute of the RAS, Leninsky prospect, 53, 119991, Moscow, Russia
A. D. Lomonosova
P. N. Lebedev Physical Institute of the RAS, Leninsky prospect, 53, 119991, Moscow, Russia
S. A. Kuzmichev
Faculty of Physics, Lomonosov Moscow State University, Leninskie Gory, 1, 119991, Moscow, Russia
K. S. Pervakov
P. N. Lebedev Physical Institute of the RAS, Leninsky prospect, 53, 119991, Moscow, Russia
V. A. Vlasenko
P. N. Lebedev Physical Institute of the RAS, Leninsky prospect, 53, 119991, Moscow, Russia
Abstract
The features of the I(V) and dI(V)/dV characteristics of tunneling break-junctions formed in underdoped BaFe2-xNixAs2 single crystals with x ≈ 0.07 and 0.08 and critical temperatures Tc ≈ 16 and 19 K, respectively, were studied at various temperatures. A reproducible nonlinearity of the I(V) characteristics and tunneling conductance was observed both below and above Tc. This behavior is unusual for contacts based on conventional superconductors and is not directly related to superconducting properties. The nonlinearity is characterized by the presence of maxima and minima in the dI(V)/dV-spectra located at bias voltages Vmax ≈ 21 mV, Vmin ≈ 40 mV for compounds with x ≈ 0.07 and Vmax ≈ 19 mV, Vmin ≈ 48 mV for x ≈ 0.08. Below Tc, features caused by incoherent Andreev reflections are observed on the background of this nonlinearity. The energy parameters of the superconducting state were estimated from the analysis of the Andreev features of dI(V)/dV spectra: the magnitude of the large superconducting gap 2ΔL(0) = 5.8–7.9 meV and the small superconducting gap 2ΔS(0) = 2.3 meV for BaFe1.92Ni0.08As2 compounds, as well as the large superconducting gap 2ΔL(0) = 4.5–8.5 meV for BaFe1.93Ni0.07As2 (the range of the 2ΔL(0) values relates to possible anisotropy of the superconducting condensate properties in k-space). The origin of the observed effects is discussed.
Keywords: high-temperature superconductivity, pnictides, tunneling spectroscopy, electronic density of states
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